Recovery of ethylene



Feb. 23, 1954 A. P. LlEN ET AL RECOVERY OF ETHYLENE Filed June 29, 1951 INVENTORS: CARL E. JOHNSON ARTHUR P. LlEN ATTORNEY:

PODOOKQ mZwJ LOmm HHddlHiS PE 93 "5 822 mnlmiz Patented Feb. 23, 1954 UNITED STATES PATENT OFFICE RECOVERY OF ETHYLENE Arthur P. Lien, Highland, and Carl E. Johnson,

Griflith, Ind., assignors to Standard Oil Company, Chicago, 111., a corporation of Indiana Application June 29, 1951, Serial No. 234,415

Claims.

- cation Ser. No. 226,945.

Ethylene is a notably useful constituent of such light hydrocarbon gases, for example, as

are produced by the intensive cracking of petroleum stocks. It has also been produced'by partial oxidation of ethane. Those fractions of hydrocarbon gases which contain ethylene contain also propylene, traces of higher olefinic homologs and varying proportions of saturated hydrocarbon gases. The separation of ethylene from these other constituents, for such purposes, for example, as the preparation of polyethylene, is particularly difiicult and expensive.

A primary object of the present invention is the provision of a process for the recovery of ethylene from gases that contain ethylene, homologous olefinic hydrocarbons, and saturated hydrocarbons boiling within the same range. other object of the invention is the provision of a process for the recovery of ethylene and propylene from refinery gas streams or the like containing'the same. Yet another object of the invention is the provision of a process for the separation of ethylene from propylene and saturated gases and the simultaneous conversion of propylene to an ester or polymer. The invention has for other objects such other advantages or results as will be found to obtain in the specification or in the claims hereinafter made.

We have now accomplished the foregoing objects by providing a process which, briefly stated, comprises the steps, in combination, of bringing a gas stream containing saturated hydrocarbons, ethylene-and one or more higher boiling olefins into contact with an organic sulfonic acid to esterify the higher boiling olefins or catalytically to effect their polymerization, contacting the remaining mixture of ethylene and parafiinic hydrocarbons with an organic sulfonic acid-boron 'trifluoride complex to esterify the ethylene, dis- 'and any excess sulfonic acid and thermally decomposing the ethyl ester to liberate substantially pure ethylene. Included withinthe scope of the invention are improvements in process, such, for example, as regeneration of catalysts, esteritying agents, and the like. For the pupose of brevity these elements of the process combination will be hereinafter described in conjunction with a description of an operation of the process of the invention.

The feature, per se, of separating propylene from a gas stream containing propylene and ethylene is disclosed and claimed in U. S. 2,576,535. Organic sulfonic acids having an acid strength substantially equivalent to an alkanesulfonic acid having less than seven carbon atoms per molecule and being in a concentration of at least weight percent, including methane-, ethane-, propane-, butanesulfonic acids, or mixtures of these, and benzene-, naphthalene-, or alkyl naphthalenesulfonic acids, are useful in effecting the polymerization of propylene and, under different conditions, have the property of esterifying the propylene. Alkanesulfonic acids or mixtures thereof having not more'than seven carbon atoms per molecule are preferred since by their use within prescribed temperature limits substantially pure propyl esters or propyl polymers of selected size can be produced. The arylsulfonic acids, and sulfuric acid that is diluted to about 70% concentration, will selectively remove propylene from the gas stream but are not equivalent to the alkanesulfonio acid in the r further object of the invention of providing'substantially pure propyl esters and polymers of selected size.

Selected reaction temperatures effect different reactions in the'propylene separation step. If the temperatures are maintained below about 60 C., e. g. 10 C. to 60 C., and preferably between about 20 and 40 C., esterification of the propylene and any other secondary olefins will result. If the reaction temperature is maintained between 60 and C. and preferably at about 70 C., polymerization of the propylene and other secondary olefins will occur. At temperatures within a narrow range near the dividing point between esterification and polymerization temperature ranges, secondary esters and the sulfonic acid will react to form a mixture of the ester and polymer. Tertiary olefins such as isobutylene polymerize at ordinary temperature.

After a propylene ester and any excess sulfonic acid has been withdrawn from contact with the gas mixture, the propyl alkyl sulfonate can be heated to a temperature between about 60 and C. and the propylene will be polymerized to ,form a mobile oil, thus liberating the acid. The liberated acid from which the said mobile oil has been decanted can be recycled.- If the desired product is the propyl ester, excess acid can be neutralized and the product can be water washed. The ester can then be purified by fractional distillation. In another modification the propyl ester may be reacted with an isoparafiin (e. g., isobutane) or with aromatics (e. g., benzene) to provide valuable alkylate and at the same time to liberate the acid.

Propylene reacts with alkanesulfonic acids at a moderate rate at about 40 pounds per square inch gauge pressure and about 30 C., and it re acts somewhat more slowly at this temperature when the pressure is atmospheric. The required time of contact of propylene and alkanesulfonic acid depends considerably on the intimacy of contasting as well as on the temperature and pressure of the esterification; generally speaking, times of contact may be in the range of about ten minutes to about ten hours, th optimum time in any case being easily ascertainable by preliminary tests. Esteriiication of propylene proceeds more rapidly. than its polymerization,

and it was found that propylene-polymerizes relatively slowly even at temperatures. as high as 110 C. It is preferred, therefore, to employithe described procedureof first preparing the propyl alkanesulfonate-and thendecomposing the ester, in the presence of acatalytic amount .of'excess acid, in a separate zone.

Thereaction temperature in the ethylene esterification step is, maintained between about 20 and 75 C. andipreferably between about 20 and 40? C, ,Esterification of the ethyleneis catalyzed by a boron ,trifluoride-organic sulfonic acid complex. The concentrationof, boromtrifluoride in the complexcan vary widely from 0.5 to 50 mol percent boron-,trifluoride based on the mols of sulionic ,acid employed, with ,apreierable range of to 25 mol percent .boronetriiiuoride. The complex is preferably an alkanesuironic acid complex inwhiclrthe alkanesulionic acid. can contain from 1 to about ficarbon atomspermolecule, inclusive. It is preferredto employat least one mole. of the sulfonic acid per molof ethylene contained in thegas. stream. Theemployment of acid in excess will ensure substantiallycomplete esterification ofthe ethylene. The product ester is aliquid andis readily separable from the gas stream.

The ethylene-esterification reaction can be performed at atmospheric pressure but proceeds more readilyat elevated pressures between about and 60 atmospheres. If adequate contact between the acid complex and the. ethylene is provided, the esterification reactionis quiterapid and will proceed to completion in a fewminutes. On the other hand, if the contactis poor and :low reaction temperatu-res'such as 0 C. are employed, the esterification :will take several hours.

After formation of the ester and separation of the gas stream, boron triiluoride contained in excess acid complex-and any remaining in solution in the product is strippedfrom the product mixture by passing therethroughan inert gas at a temperature below about 80 C., the temperature above which ethylene polymerizes in the presence of boron trifluoride. After removal of boron trifluoride, decomposition of the ethylene ester and recovery of the liberated ethylene is accomplished at temperatures betweenabout 150 and about 250 C. Lower temperatures with- .in this ran e, referably temperatures. of about 150 to-l80 C.',- provi de decomposition of the ester and liberation of'the ethylene while yet 4 more completely avoiding formation of ethylene polymer.

In the drawing that is provided solely for purposes of illustrating the invention, the single figure is a schematic flow diagram of a preferred embodiment of the process of the invention, designed to separate pureethylenefrom a refinery gas'stream containing ethylene, propylene and saturated hydrocarbons boiling within the same range. For the purpose of brevity, valves, pumps, and like conventional apparatus are not shown, except where the function of such apparatus is necessary to a description of process operation.

Referring now to-the drawing, a feed gas, obtained asaxdemethanized fraction of the hydrocarbon oii-gases from a fluid catalytic cracking unit;is delivered-from a source ID by blower H through line I2 to reactor I3. The composition of the gas as shown byan approximate moi ratio of its components is as follows:

20 mols-,methane 10,0 mols ethylene- 210. mols. ethane 12.5 mols ,propylene 60 mols propane 10 ,mols .Cs hydrocarbons This mixed gas is delivered-iintoreactor' l3-at a rate of approximatelyZOOflOOcubic,feetgper hour (measured at l51C.-.and.lyatmosphere), and is passed thereindn countercurrentzcontact with down-flowing mixed alka-nesulfonic acid containing methane ethane-, .and .propa-nesulionic acids and havinggthefollowing characteristics:

M ixed alkanesulyfom'c acids:

Average molecular weight ,llO- Specific gravity (25'/4"C:); 1.30-1.35 Color Light amber Composition, weight percent- Alkanesulfonic acids 94 Water '3 H2804 3 Ash Less than 0.05

The reactoris operated at a temperature of about-30 C. and the propylenein the gasstream reacts with the. said sulfonic acidsto form propylene esters. Thealkanesulfonic acids are delivered 'at-arate of about 14,000 pounds perhour from storage vessel through linei5. into reactor l3. Propyleneester,,together, with any smallamountsofbutylene ester, is withdrawn from reactor 1 3 with ,excessacid as-a single liquid phase through line [6 into ,a separator l'iin which the solution .isheatedto between about 60 and 120 C.- and preferably to.about:70 .C. The propyl ester-decomposes to release acid and propylene,,,which polymerizesto forma mobile oil. The propylene polymer isdecantedfrom separator -H and is withdrawn therefrom through line I 8 to storage. Theexcess-acid and thereleased acid -are withdrawnirom the; separator throu h line it and recycl d through line 2i (by meansof pump 2.! toacidstoragedk-a Anybutyl ester is likewise decomposed to .givebutylene polymer; when isobutylene. is present,,its polymer may form .aseparate liquid phase-which can be withdrawn with theacid-esterphase.

Gas from which propylenehas beensubstantiallyentirely removed and which contains ethyl- .en and saturated hydrocarbons .flowsirom reacdown-flowing boron trifluoride-sulfonic acid complex. At the beginningof plant operation, this complex is formed and introduced into reactor 23 in the following manner. Alkanesulfonic acid is delivered from storage vessel I4 through a line '24 at a rate of about 14,000 pounds per hour. The

acid mixes and forms a complex with boron tri- I fluoride that is delivered at a rate of about 700 pounds per hour from storage vessel 25 through line 26. As operation continues, the catalyst complex will be recycled to storage vessel 25 and only make-up complex need be produced.

Ethyl alkanesulfonate is formed in reactor 23 'by reaction of the ethylene contained in the introduced gases with the alkanesulfonic acid of the complex in the presence of the catalytic boron trifluoride at a temperature of about 40 C. The so-formed liquid ethyl alkanesulfonate and any excess boron trifluoride-sulfonic acid complex are withdrawn from the reactor through line 21 and are delivered by pump 20 to stripper column 29.

Light saturated hydrocarbon gases from which ethylene and propylene are substantially entirely removed flow at a rate of about 16,000 feet per hour from reactor 23 through line 30 to the juncture of valved lines 3| and 32 in each of which lines a predetermined proportional amount of the hydrocarbon gases is directed. Hydrocarbon gases from line 32 are introduced into stripper 29 at a point near the base of this column. These hydrocarbon gases rise in countercurrent contact with downflowing liquid comprising ethyl ester, excess boron trifluoride-sulfonic acid complex, and any dissolved boron-trifluoride, which liquid is maintained in the stripper at a temperature less than about 80 C. Under these conditions the rising hydrocarbon gases strip out substan tially entirely the boron trifluoride content of the liquid. Liberated boron trifluoride passes overhead through line 33 and is introduced into the stream of boron trifluoride flowing from storage vessel 25 in line 26. A small volume of saturated hydrocarbon gases will be recycled with the boron trifluoride and will be liberated therefrom in the reactor 23.

A portion of the light hydrocarbon gases is directed through valved line 3i into a scrubber 36 wherein the said gases rise in countercurrent contact with downflowing liquid alkanesulionic acids at a temperature of -l to 50 C., preferably to 35 C. The alkanesulfonic acids are delivered from storage vessel [4 through line 31 into the scrubber 3E. Boron trifluoride liberated from the complex with sulfonic acid in the reactor 23 is absorbed in the downflowing sulfonic acids in the scrubber 36.

Sulfonic acid containing absorbed boron-trifluoride as a complex in the acid is withdrawn from the scrubber 36 through a line 30 and is pumped by a pump 33 through the line 39 into the storage vessel 25. In the course of preparing boron trifluoride-sulfonic acid complex for the esterific-ation of ethylene, it is observed that not i only are substantially all traces of boron-trifluoride removed from the eiiiuent saturated hydrocarbon gases, but also any loss of boron trifiuthe stripper 20 and are pumped by pump et into stripper 43. Because of the factthat'boibn trifluoride has been substantially entirely removed from this liquid product containing the ethylene:

merization of the ethylene.

ester and sulfonic acid, -temperatures between 150 to 250 C. can be employed to decompose the ester without encountering substantial poly- As hereinbeiore noted, the lower temperatures within the range and preferably temperatures between 150 to 180 C. inclusive, will eifect decomposition of the ethyl ester while encountering the least possibility of any ethylene polymerization. The so-liberated ethylene flows at a rate of about 36,000 cubic feet from the stripper 43 through a line 44 to storage utilization means, not shown.

Alkanesulfonic acid is withdrawn from the base of the said stripper 43 through line 45 to the juncture of valved lines 46 and 41. That portion of the alkanesulfonic acid which flows through line 41 is directed into a regenerator 48. Alkanesulfonic acid in regenerator 48 is heated to remove highly volatile contaminants which are discharged through line 49. The regenerator 48 is generally operated under a vacuum provided by apparatus not shown. Accumulated sludge is withdrawn from the base of the regenerator 48 through a discharge line 50. Regenerated alkanesulfonic acid is removed as a sidestream from regenerator 48 through a line 5!, from which the regenerated acid flows into line 20 and is recycled by pump 2l to acid storage H4.

The remaining portion of the alkanesulfonic acid withdrawn from the stripper 43 flows through valved line 46 and through the line 20 and is also recycled by pump 2! to acid storage vessel l4. Crdinarily, only a small portion of the withdrawn acid need be regenerated in order to maintain the quality of the reactant sulfonic acid. Variations in operatingconditions will, of course, vary the life of'the employed sulfonic acid.

The process herein described presents numerous advantages over known processes for ethylene concentration. For example, the costly step of low-temperature fractionation to segregate an ethylene-ethane streamfrom lowerand higherboiling gases is eliminated. Furthermore, our direct removal of ethylene from saturated gases by chemical reaction to form the ester is much more efficient than processes involving oil absorption or hypersorption.

Having described our invention, we claim:

' 1. A process of recovering substantially pure ethylene from a gas containing ethylene, propylene, and other constituents boiling within substantially the same temperature range, the said process comprising: scrubbing the said gas with a sulfonic acid thathas an acidity substantially equivalent to that of a low-molecular weight alkanesulfonic acid in at least weight percent concentration while maintaining a temperature between 20 and C, and effecting removal of propylene from the gas; passing substantially propylene-free gas from the said scrubbing step into direct contact with a sulfonic acid containing from 0.5 to 50 mol percent, based on the acid, of boron trifluoride at a temperature below about 75 C.,' and esterifying ethylenecontained in the pyleneand saturated hydrocarbons; boiling-with- -'inthe same temp'e'ratu-rer'ange; the said process comprising: introducingithe said gas, intoidi'rect memes;

7 contact with ana'cid selectedrfroma group consisting of a low molecularweight alkanesulfonic acid, arylsulfonic acids and an approximately 70- percent-concentrated sulfuric acid while maintaining the temperature of "the gas and acid between 20 and 120 C. and effecting removal of propylene from the gas; passing the gas from the propylene-removal step into contact with an acid complex consisting of an alkanesulfonic acid having less than seven carbon atoms per molecule and from 0.5 to 50 mol percent, based on the acid, of boron trifluoride while maintaining a tempera ture below about 75 C. and thereby esterifying ethylene contained in the gas; separating from the gas stream a liquid product comprising soformed ethyl ester, boron trifiuoride complexes and unreacted acid; heating the said liquid product to a temperature below 80 C. and stripping dissolved boron trifiuoride from the ester; and heating the said ethyl ester, free of boron trifluoride, to a temperature between about 150 and 250 0., thereby liberating substantially pure ethylene.

3. A process of recovering substantially pure ethylene from a gas containing ethylene, propylene, and saturated hydrocarbons boiling within the same range, the said process comprising: introducing the said gas into direct contact with an alkanesulfonic acid having less than seven carbon atoms per molecule while maintaining the reaction temperature between 60 and 120 C. to polymerize propylene contained in the said gas; withdrawing liquid propylene polymer and the alkanesulfonic acid from contact with the gas; passing substantially propylene-free gas-from the polymerization step into contact with an acid complex consisting of an alkanesulfonic acid having less than seven carbon atoms per molecule and from 0.5 to 50 mol percent, based on the acid, of boron trifiuoride while maintaining a temperature below about 75 C. and thereby esterifying ethylene contained in the gas; separating from the gas stream a liquid product comprising so-formed ethyl ester, boron trifluoride complexes and unreacted acid; heating the said liquid prod uct to a temperature below 80 C. and stripping boron trifluoride therefrom; and heating the ethyl ester, free of boron trifluoride; to a temperature between about 150 and 250 0., thereby liberating substantially pure ethylene.

4. A process ofrecovering substantially pure ethylene from gas streams containing ethylene, propylene and saturated hydrocarbons boiling within substantially the same temperature range, the said process comprising: introducing the said gas into direct contact with an alkanesulfonic acid having less than seven carbon atoms per molecule while maintaininga temperature between about 10 and 60 0., thereby esterifying propylene contained in the gas stream; withdrawing so-formed liquid propyl alkanesulfonate and any unreacted acid fromthe gas stream; passing propylene-free gasfr'om. the propylene-esterificationizone into'direct contact with an acid complex consisting of an alkanesulfonic acid having less than seven carbon atoms per molecule and from 0.5 to 50 mol percent, based on the acid, of boron trifiuoride while maintaining a temperature below about 75 C and thereby esterifying ethylene contained inthegas; separating from the gas stream so-iormedliquidethyl ester, borontrifiuoride complexesand unreacted acid,

.heating the. said liquid; product ,to a temperature below 80 C. and stripping therefrom boron .trifluoride; and heating the said; ethyl ester, free 8 of boron trifl'uoride, to; a: temperature between about 150andJ250 0., thereby liberating substantially pure? ethylene;

5. The process ofclaim 4 in which the soformed propyl alkanesulionate and unreacted acid are heated to a temperature between about 60 and 150 C. to polymerize propylene contained in the withdrawn propyl alkanesulfonate and liberate the sulfonic acid from the ester.

6. A process of recovering substantially pure ethylene from a gas stream containing ethylene, propylene and saturated hydrocarbons boiling within substantially the same range, said process comprising: introducing the said gas continuously into direct contact with at least one mol, per mol of propylene in the said gas, of an alkanesulfonic acid having less than seven carbon atoms per molecule while maintaining a reaction temperature between about 10 to 60 C. to esterify the said propylene and form a propyl alkane sulfonate; Withdrawing the liquid propyl alkane sulfonate and any excess acid from contact with the said gas; passing substantially propylene-free gas at superatmospheric pressure into contact with at least one mol of an alkane sulfonic acid, having less than seven carbon atoms per molecule, per mol of ethylene contained in the gas and from 0.5 to 50 mol percent, based on the acid, of boron trifluoride while maintaining a temperature below about C., and thereby esterifying ethylene contained in the gas; separating from the gas stream a liquid product comprising so-formed ethyl ester, boron trifiuoride complexes and unreacted acid; heating the said liquid prodnot to atemperature below C. and stripping boron trifiuoride from the ester; and heating the said ethyl ester, free of boron trifiuoride, to a temperature between about 150 and 250 (3., thereby liberating substantially pure ethylene.

7. A process of recovering substantially pure ethylene from'a gas containing ethylene, propylene and saturated hydrocarbons boiling within substantially the same temperature range, the said process comprising: introducing the said gas into direct contact with an alkanesulfonic acid having less than seven carbon atoms per molecule and a concentration of at least weight percent while maintaining the temperature of the gas and acid between 20 and C. and efifecting removal of propylene from the gas stream; passing substantially propylene-free gas into contact with more than one mol of an alkanesulfonic acid, having less than seven carbon atoms per molecule,

per mol of ethylene contained in the gas, and from 0.5 to 50 mol percent, based on the acid, of boron trifiuoride while maintaining a temperature between about 20 and 40 C. and thereby esterifying ethylene contained in the gas; separating from the gas stream a liquid product comprising so-formed ethyl alkane sulfonate, boron trifluoride complexes and unreacted acid; hea ing the said liquid product to a temperature below 80 C. and passing through the product an inert gas consisting of propyleneand ethylene-free saturated gas from the said gas stream to strip boron trifiuoride from the ester; and heating the said ethyl ester, free of boron trifluoride, to a temperature between about and 250 0., thereby liberating substantially pure ethylene.

8. A process for recovering substantially pure ethylene from gas containing ethylene, propyl- T ene, and saturated hydrocarbons boiling within "substantially thesame ternperature range, the j said process comprising: introducing the said gas into direct contact with an alkanesulfonic acid having less than seven carbon atoms per molecule while maintaining a reaction tempera ture between about and 40 C. to esteri'iy propylenes contained in the said gas; withdrawing so-formed propyl all-zanesulfonate and unreacted acid from contact with the gas stream; heating the withdrawn product including the propyl alkanesulfonate to a temperature between about 60 and 150 C. to polymerize propylene contained in the withdrawn ester and liberate alkanesulfonic acid therefrom; recycling alkanesulfonic acid to the propylene esterification step; passing substantially propylene-free gas at a pressure be tween about 10 and 60 atmospheres into contact with an acid complex consisting of an allcanesuh fonic acid having less than seven carbon atoms per molecule and from 5 to mol percent, based on the acid, of boron trifluoride while maintaining a temperature between about 20 and 40 C. and thereby esterifying ethylene contained in the gas; separating from the eiiluent gas stream, that is substantially ethyleneand propylene-free and contains only saturated. hydrocarbons, a liquid product comprising so-formed ethyl alkanesulfonate, boron trifiuoride complexes and unreacted acid; heating the said product to a temperature just below about 80 C. and passing through it at least a portion of said efiiuent gas to strip boron trifiuoride from the ester; flowing said effluent saturated gases containing boron trifluoride into contact with alkanesulfonic acid, having less than seven carbon atoms per molecule, to form the said acid complex; recycling the so-- formed complex to the ethylene-esterification step; heating the said ethyl ester, free of boron trifiuoride, to a temperature between about and 180 0., thereby liberating acid and substantially pure ethylene; and recycling so-liberated acid to prior process steps utilizing acid.

9. In a process for separating substantially pure ethylene and propylene from a gaseous stream which consists of ethylene and propylene admixed with paraffinic hydrocarbons and in which propylene is first separated from the gas stream by contacting the gas with a sulfonic acid and separating the so-formed propylene reaction product, the combination with said propylene recovery of intimately contacting said stream with concentrated alkanesulfonic acid in the presence of between 0.5 and 50 incl percent, based on the acid, of boron trifiuoride while maintaining a temperature below about C. and thereby esterifying ethylene contained in the gas to form an alkanesulfonic acid-ethylene ester, introducing liquids from said contacting step into a stripping zone and therein removing boron trifiuoride and unreacted hydrocarbons from an allranesulionic acid-ester mixture at a temperature below C., and heating the stripped liquid after re" moval of boron trifiuoride therefrom to convert said ester into substantially pure ethylene and allranesulfonic acid.

10. The process of claim 9 in which the said stripped liquid after removal of boron trifluoride is heated to a temperature between about and C.

ARTHUR P. LIEN. CARL E. JOHNSGN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,065,540 Schneider Dec. 29, 1936 2,257,297 Grooinbridge Sept. 30, 1941 2,393,895 Fleming Jan. 29, 1946 2,403,010 Wadley et al Sept. 2%, i946 2,51'6535 Proell Nov. 27, 1951 OTHER REFERENCES Booth et al., Boron Triiiuoride and its Derivatives (Wiley and Sons, N. Y.) 1949, pages 198-200 and 205. 

1. A PROCESS OF RECOVERING SUBSTANTIALLY PURE ETHYLENE FROM A GAS CONTAINING ETHYLENE, PROPYLENE, AND OTHER CONSTITUENTS BOILING WITHIN SUBSTANTIALLY THE SAME TEMPERATURE RANGE, THE SAID PROCESS COMPRISING: SCRUBBING THE SAID GAS WITH A SULFONIC ACID THAT HAS AN ACIDITY SUBSTANTIALLY EQUIVALENT TO THAT OF A LOW-MOLECULAR WEIGHT ALKANESULFONIC ACID IN AT LEAST 90 WEIGHT PERCENT CONCENTRATION WHILE MAINTAINING A TEMPERATURE BETWEEN 20* AND 120* C. AND EFFECTING REMOVAL OF PROPYLENE FROM THE GAS; PASSING SUBSTANTIALLY PROPYLENE-FREE GAS FROM THE SAID SCRUBBING STEP INTO DIRECT CONTACT WITH A SULFONIC ACID CONTAINING FROM 0.5 TO 50 MOL PERCENT, BASED ON THE ACID, OF BORON TRIFLUORIDE AT A TEMPERATURE BELOW ABOUT 75* C., AND ESTERIFYING ETHYLENE CONTAINED IN THE GAS; REMOVING THE SO-FORMED ETHYL ESTER FROM THE GAS; STRIPPING BORON TRIFLUORIDE FROMTHE ESTER AT A TEMPERATURE BELOW 80* C.; AND HEATING THE ETHYL ESTER, FREE OF BORON TRIFLUORIDE, TO A TEMPERATURE BETWEEN ABOUT 150* AND 250* C. THEREBY LIBERATING SUBSTANTAILLY PURE ETHYLENE. 